Recent nonaqueous electrolyte secondary batteries utilized as main power supplies for mobile communication equipment and portable electronic equipment have high electromotive forces and high energy densities as their characteristics. Examples of material for a positive electrode active material used in the nonaqueous electrolyte secondary batteries include lithium transition metal composite oxides, such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and the like, for example.
In the nonaqueous electrolyte secondary batteries (i.e., lithium ion secondary batteries) using a positive electrode active material containing a lithium transition metal composite oxide, an increase in charge end voltage has been examined. The term, “charge end voltage” herein means a maximum value of the charge voltage at which a battery can charge up safely. This enables provision of high-capacity nonaqueous electrolyte secondary batteries.
In view of this, in the nonaqueous electrolyte secondary batteries, the use of a positive electrode active material, in which two kinds of lithium transition metal composite oxides are mixed as a positive electrode active material, has been proposed (see, for example, Patent Document 1). This attempts to increase the charge end voltage (4.3 V or higher, for example) without lowering charge/discharge cycle characteristics and thermal stability.
Patent Document 1 concludes that a factor that contributes to lowering of the charge/discharge cycle characteristics (i.e., a factor that contributes to a decrease in battery capacity in association with repetition of a charge/discharge cycle) in a nonaqueous electrolyte secondary battery using a positive electrode active material containing LiCoO2 and having an increased charge end voltage of 4.3 V or higher might be decomposition of a nonaqueous electrolyte or breakage of the crystal structure of LiCoO2.
In view of this, the technique disclosed in Patent Document 1 uses a lithium transition metal composite oxide of LiCoO2 containing at least both Zr and Mg as one of the two kinds of lithium transition metal composite oxides of the positive electrode active material in which the lithium transition metal composite oxides are mixed. By doing so, decomposition of the nonaqueous electrolyte or breakage of the crystal structure of LiCoO2 is decreased as an attempt to prevent lowering of the charge/discharge cycle characteristics. Further, a lithium transition metal composite oxide having high thermal stability is used as the other lithium transition metal composite oxide. This may prevent lowering of thermal stability.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-164934    Patent Document 2: Japanese Unexamined Patent Application Publication No. 5-182692